I see some job ads for AMC have a primary AC such as an EC135 and secondary as a EC130 while others have no secondary. Is the secondary mainly used a back up for when the primary is down for maintenance?  

On bases that have no secondary, what happens when that EC135 or Bell 429 is down for major maintenance or an annual? Do they bring in a replacement AC or does the crew just sit around and do nothing, or do they get assigned busy work? Thanks in advance.

The secondary is, indeed, a backup that's just not the same aiframe type.  For most regions, the spare aircraft is the same aircraft type (although may not have the same cockpit setup so they make sure you have differences training to know how to use the SAS or other system across all cockpits of that aircraft type). 

You'll usually get a spare pretty quickly if your bird goes down hard for mx.  Just depends on if the spare is taken.  If not; you come in for your usual shift and hang out (knock out online training, clean the base, etc.).  

They have a bunch of dedicated back-up aircraft accross each region. If down for scheduled mx the back-up is provided. For unscheduled mx they try but first come first served. I've seen they take the back-up from a 4 aircraft program and send it to a 1 aircraft program in some cases. Poor back-up aircraft get no love.

As stated above, regions have float aircraft to use at bases when their primary is out of service for major work. The problem is that float aircraft get no maint love, so they are pretty beat up. And, it may be a different model than the one they fly. I delivered an old 407 basic to a base that flew a 407 GX as their primary. They were like deer in the headlights. 

As previous post mentioned, you will have two broke aircraft at same time, by yourself most likely. Better get your money up-front because HEMS is a JOKE

Close together, give coverage in the case that one base is down for whatever reason.

If 2 differen AC one may be the only type spare available. We had an EC130 (only singles in state) so we had to be dual rated in the 135 since they were the only spares. As other have mentioned with the different models...switched vendors, had 3 P2+ at the bases waiting on a like spare. Until then we had a spare that was CDS..= old, totally different display layout.  "Here's the spare"...It's a 135, figure it out. 

Thanks for all the great answers and responses. This answered my question.