in/out trunks setup

I'm hoping there is an easy solution to this, but heres my situation.

I have 6 in/out SIP channels that my DIDs ride in on I also get free outbound local calling on these channels. These are my primary channels.

I also have 4 outbound channels with another vendor which has better LD rates than my primary six. These are my LD channels.

OK it is real easy to route LD to the 4 outbound LD channels, that is a no brainer.

But, what I'd like to do with local outbound calls is have them go to my primary 6 channels until 4 channels are being used, then switch to dialing out on my LD channels. That way I will always have 2 channels open for inbound calls.

I was thinking I could create 2 trunks with the same credentials, one with 2 channels and one with 4 channels. Then when setting up my local outbound routes, I only use the trunck with 4 channels assigned.

Will this accomplish what I'm looking to do? Is there some other way to do this?