@Giuseppe Larosa makes an excellent point about the "particular" form of "maximum-paths" needed to work with iBGP. If you're in the situation where it's not supported, you might also consider using perhaps eBGP between your core routers and CEs or perhaps an IGP (maximum-paths 2 more likely to work, with the former [correct Giuseppe?] and the latter, generally with Cisco, defaults to maximum-paths 4).
Giuseppe raises another point, if doing something like prepending, out, you'll need to adjust that. Ditto if using any other "traffic engineering" to "steer" ingress traffic.
Also, depending on how your two MPLS vendors implement their internal (private) BGP topology, you may see a difference there too you need to adjust for. (Years ago, I worked at a company doing MPLS/BGP/VPN, internationally across two different vendors. The one presented the whole world as one AS, the other had regional ASs. So the former, for inter-region communication, always appeared to have less "hops" on the one vendor than the other.)
Lastly, if your really want to use the two paths "efficiently", then you want to run something like Cisco's PfR on your CE routers. Maximum-paths just, more-or-less, round-robins flows to "equal" cost destinations. PfR dynamically moves flows about to load balance actual utilization. It can also do this unequally. It can also deal with "impairments" on one provider and shift to another. It can also shift flows based on QoS considerations. For example, you only show two sites, but if you had three, or more, how do you insure two sites sending to another, not only load balance their links to the MPLS clouds, but the egress on the links at the destination site? PfR can do that too.
