After an individual has Mekayem the Mitzva of Tefillin by Tefillat Shachrit, is there an issue to wear only one Tefilla (either Yad or Rosh) afterwards to learn, etc.., or must he wear both (or neither)? Thank you.
There is an issue to wearing the shel rosh without the shel yad when one has both of them available. This is one of the reasons that we put the yad on first and take it off last, so that we will always have the shel yad on our hand while the shel rosh is on our head. (If someone only has the rosh, he may put it on- see O:CH 26-1). Regarding to wear the yad without the rosh, I don’t see any problem with it.
Minachos 36a, O:CH 28-2, Rivivon Hilchos Tefilliin pg. 442, Bur Halacha siman 26 D:H Ma’niach. Note: According to the Eshel Avrohom 28 (Butshash) there may be a preference not to talk out between taking off the two teffilin, however if this will cause that you not to wear the second one, don’t be makpid on it.