I would recommend you ask your question at Sky and Telescope's website.

Unfortunately, I do not have the technical knowledge to confidently answer your question myself, but those working at S&T and/or their readers certainly do.

Hasitha,

There is absolutely nothing wrong with using two 'scopes. Pick a few stars and use them as standard candles in the two telescopes. Observe the same stars with different filters and do a little mathematics to obtain the transfer function from the CCDs. That calibrates your detectors, and then you can observe the variable star at two wavelengths.

I don't see any obstacle to having photometrically absolute data in both observing wavelengths - and it beats swapping filters.

Which star are you trying to observe and what is its period?

Why stop at 2 telescopes? I bet plenty of people would be happy to join you and observe the star from different locations around the world. Maybe you could even arrange for simultaneous or continuous observations. I don't know which is more useful for roAp stars......  what is the goal of your study?

As James Gary already explained there will be no problem cross-calibrating among different instruments. Particularly if you have several stars in the same field of view as your science target. Ideally identify a dozen or so in published star catalogs to obtain their magnitudes in the filters that each telescope used.

Thank you sir,

I'm observing HD19918.  One pulsation period is about 19 minutes. Exposure time I applied for U and B filters was 60s and for others 30s. But since the pulsation period is so short I could take only few images within a period.

  My original research is "Determination of Oscillation periods of Delta Scuti stars using photometry". But I also chose a roAp star for asteroseismic study.I think it would be great if there were 4 telescopes observing the star at the same time because then we can use UBVR 4 filters respectively. But I'm new to astrophotography and observational astronomy.

This functionally is no different than any observing project, whether you use multiple telescopes simultaneously or sequentially. We use standards for exactly that reason. The magnitude I obtain and publish has to be reproducible by others with different instrumentation. That can be tricky, but there are plenty of standards to use (often in the same field as the target star: look at the AAVSO APASS standards).

Timing becomes important when working on combining data from different instruments taken at the same time, but that is not difficult with modern equipment. I do work using simultaneous spectroscopic and photometric observations for variable stars, albeit with targets of much longer periods.

One question. Is there a reason you are trying to get data in more than one filter while looking at asteroseismology? It looks like many studies choose to stick with just B, since these are hot stars. For example this HUGE study: http://adsabs.harvard.edu/abs/2005MNRAS.358..651K

Good luck with the project.

As you examine a periodic process, there is no need to complicate the process of observations using several telescopes. You just need to ensure that the frequency of observational cycle do not interfere with the period of the star. In one night, all four of your phased light curves will be filled by observational points evenly. I see another problem. In the vicinity of your star there are no appropriate in terms of color-index comparison stars. All close and relatively bright stars are red. Your observations in the ultraviolet are burdened with significant errors. Here I am not talking about a differential extinction for stars with different colours, located on the same air mass - it is another problem. So you have to make a special efforts to get from your observations maximum accuracy and you can not abandon it by using several instruments.

There is not problem in using two different telescopes observing simultaneously the same star. As it was pointed out, several projects used this technique before, for instance, to obtain photometry of a star continuously with telescopes placed at several longitudes around the world, in order to increase the time coverage (asteroseismology studies), or even to complete light curves with special features (long eclipses, outburst,...).

Assuming that you have a network of reference/standard stars to observe, you must deal with small differences in the data sets caused by the small differences in filter pass-bands, detector spectral sensibilities, pixel sizes, and optical transmissions for each system. All of these differences among the observational setups would result in systematic effects among your light curves, and you must remove them in order to combine the light curves and not to obtain weird results. Usually, systematics can result in offsets between combined light curves in the same filter, or wrong color indices if you combine light curves with different filters for each data set.

If you are doing differential photometry you want to use the same reference stars with the two telescopes to measure those systematics and to remove them. If you are doing absolute photometry, those systematics must be absorbed in the phometry zero-point. In any case you must obtain a way to convert the data from one system to the other.

For short, yes, you can use two telescopes if you reduce carefully your data.