Force two different VoIP trunks to use two different routers

I’ve two different ADSL connenctions with two different, separate, routers.
I’ve configured two SIP trunks with two different VoIP accounts.
Is there any way to force the first SIP trunk to use the first ADSL connection (and the first router) and the second SIP trunk the second ADSL connection (and, than, the second router)?
By this way I’d can place two simultaneously SIP calls even if the upload speed of each ADSL connection is relatively poor.