I will be doing ryff's psychological well being scale's psychometric properties. Could I administer alternate form reliability simultaneously? (42-item and 18-item)
If you can get by the problem clearly identify by David Morgan, 82 total items is not extremely problematic unless the items require a great deal of time each to respond. If you wish to go ahead and do it, first, do not put the corresponding items adjacent to each other. My suggestion would be to have two forms of the 82 item scale. For half of sample, put the items on Form 1 first and the items on Form 2 second. Reverse that for the other half and randomly administer the full 82 items. This way, you would remove the order effect or at least be able to empirically verify if there was one.
As what i understood on alternate forms, the alternate instrument should be adminsitered days after the first scale was administered. So, is it practical to do it if the 18-item test is a subscale of 42?