Larry J wrote:
There will be a lot of time when the satellites can't see the rovers amd the rovers may be contending with one another for relay capacity...

That is precisely why during the mission design phase that there was an operational constraint stating that the two rovers must be separated by a surface arc of at 37 degrees (viz., so that one orbiter does "see" two rovers at the same time). At any rate, mission planners estimate that orbital relay (via Mars Odyssey and MGS) may be able to return half the MER science data, though operationally mission critical data will use the DTE links.