You can use other distributors at the same time as you use Amuse. But there are only a couple of reasons why you should:
Transferring to Amuse - If you are distributing the same song through us and another distributor, please ensure that you set the region and store settings so there is no overlap (unless you are switching to Amuse). You can't have the same track, on the same product* and in the same store for a longer period of time than to transfer from one distributor to another (where we recommend you to oversee the release few days to see that streams transitions smoothly. Read more about transferring to Amuse here: How do I switch to Amuse?
New stores - There are some small but still important stores that Amuse doesn't deliver to. Beatport is, for example, important to Djs, so feel free to use another distributor for all stores Amuse doesn't deliver to.
Previous work - you can keep old releases with the previous distributor while using Amuse for the new stuff.
*It is fine to have two distributors if you have a track on a single with one distributor and add the same track to an album with us (using the same ISRC code, of course) as the products are different. But the ISRC code must be the same if it is the same recording.