In UMTS, we had a terminology called 'Combined Registration'. It means that UE simultaneously performs registration for CS network and PS network. LTE is bascially packet only (PS only).
Now coming to the question of Why do we need this kind of simultaneous registration for two different network ? It is mainly for CS-Fallback. Unless a system operator/UE use Voice Over IMS, we use legacy network (e.g, WCDMA, GSM, C2K) for voice call. In this case, UE has to go through the registration for the legacy network. The idea of Combined Attach is to perform the attach process for LTE and Legacy Network simultaneously.
Now coming to main question if handover is possible without combined attach, answer is yes for PS but for CS it is not.