Since the MXs cannot do LACP generally speaking you would not connect multiple links from a single switch to a single MX. If you do do this you're subject to the rules of STP in handling loops. The MX will transparently pass BPDU's received on one link to another link (i.e. a switch may receive its own BPDU back on a different interface).
An alternative would be to make the links L3 instead of L2. You could create multiple transit networks, one per link, and connect those between your stack and the MX. Granted, the MS can;t do routed ports, but you can mimic the functionality with a single access port and an SVI.