I've heard that the heat rate depends on the least efficient beam. You have three beams, one heats at .8, another at 1, and a third at 1.7, the roid will heat at 1.7 if all three beams are used. But I could always be wrong.

If multiple beams only heat the roid at the same rate 1 beam would, I think it ought to be the average heat rate of the beams, not the least efficient or the first to start mining.

But I'm not conviced that's the case. I'm pretty sure that the times I've been group mining, the 'roids heated up faster than when I was soloing.

[edit - at about the same time sarahanne posted her reply, noting the same thing]
So I was just searching to see if there had ever been a dev post w/ details on mining heat rates (yeah, I know, bad me, should have searched before posting), and I couldn't find anything, but I did find this, that was interesting: apparently multiple players only get the heat efficiency bonus if they're grouped.
http://www.vendetta-online.com/x/msgboard/2/9202
[/edit]

but not as fast as they would if you all had been mining the roid together without grouping.