If you have two separate loops of tracks (forget about "blocks" or anything else), and you have each loop set up to be controlled separately by a DCS Remote Commander set (one handheld remote and one receiver for each loop), you will be able to use just one of the remote handhelds to control either or both of the loops.
HOWEVER, you will want to space the two receivers far enough apart, or "isolated" from each other in some way so when you do transmit a signal from the handheld remote to one loop's receiver it will not also be read simultaneously by the other loop's receiver.
The handheld remotes and receivers are the same in every starter set--they are not unique to any particular set in terms of the signal they send or any special coded identification in the engine. For example, I use just one DCS Remote Commander set on my tinplate layout, and that one set (one receiver and one handheld) will operate any MTH starter set locomotive that has not been "altered" from the factory default setting.
Since the DCS remote Commander handled units use an infrared signal to "talk" to the receiver, and since that infrared beam is fairly close to being line-of-sight, you should not have a problem if the two receivers your are using are spaced far enough apart. The truth be told, that line-of-sight thing is one of the things I don't particularly like about the DCS Remote Commander because I sometimes tend to forget that I must point the handheld at the receiver. It takes some getting used to because if you point it in some other direction, the receiver won't respond (blinking light) and, as a result, the locomotive just goes on its merry way.
