When should one doTula snanam? Is it before performing pratha snanam or during madhyanika snanam before sandhya vandanam? Or should it be done during both these periods?

Tula snanam can be done while doing pratha snanam before morning sanddya, there is no need to do it two times.
It can be done before madhyanika snanam but one has to remain without partaking food. If one takes a bath during madhyanikam, tula snanam can be done prior to this snanam.