It is forbidden to speak between putting on the shel yad and the shel rosh. Likewise, a person should not take an unnecessary break between donning the two tefillin. One should not point or make motions during this time, either. If one did speak, the halacha is as follows: According to the opinion that only one brocha is generally made, one should recite a second brocha of al mitzvas over the shel rosh in this case. According to the other view, which holds that two brochos are made, the brocha of lehaniach is recited again on the shel rosh in this case. Before saying the brocha of lehaniach, one should touch the shel yad and move it out of its place so that the brocha can apply to the shel yad as well. Others say that the brocha of al mitzvas should be said first. After saying the brocha of lehaniach, the shel yad should be moved out of its place and then returned.
[סעיף ט, ס"ק כח, כט ו־לב, וביה"ל ד"ה ואם וד"ה ולדידן]