If after Shachris (wearing Rashi Tefillin) one lays Rabbenu Tam Tefillin and after removing R.T. Tefillin, he wants to re-lay Rashi Tefillin, can he do that? How much time must elapse between R.T. Tefillin and Rashi Tefillin. What about after Musaf of Rosh Chodesh?
One can certainly put back Rashi Tefillin after removing the R. Tam.
After Mussaf on Rosh Chodesh there is a dispute among different opinions about putting Tefilling back on (see Rama Mi-Fano 108; Yecheveh Daas 6:6; Yabia Omer 3:53). Both ways are acceptable.
This question is of only relevant for people who wear Tefillin the whole day in general. Those who don’t wear Tefillin the whole day should not do so on Rosh Chodesh.