I think gradient ultracentrifuge is the best way, for purify hole particle, The way you say also is possible.

Most important question is what your next step?

I assume you can do inoculation of serial-diluted homogenized tissue supernatants in cultured cells in 24-well plates to get pure virus infections and propagations. I have also found an article which discusses the isolation of pure viruses from mixed infections. please find below

http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0156110

And another paper to isolate NDV virus

http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-89132016000100332

I hope this information is helpful to you. Goodluck

I think it is possible to add NDV antisera to the sample contains both viruses after that added the washed Rbcs to catch H2N9. Do centrifugation and get the supernatant with NDV..

Different methods could be carried out as mentioned previously gradient ultracentrifuge (hard to perform), but the easiest in my opinion ,depending on the virus load, to purify NDV through the implementation of plaque assay using LMH cell lines if available. Otherwise you could perform egg inoculation of 1:1000 diluted sample (after incubation of your sample at 37C with H9N2 antibodies), and the other way around to get the H9N2.

I just have done it just dilute the virus. 1:1000 and dilute the Sera 1 :4 add equal amount of diluted virus to the serum incubate 2 hours inoculate the eggs and repeat it twice