It depends but likely in both. If both contracts come out of the same bank account within a few days then yes, you will fail to pay and "The process" kicks in. It does have a dependency if you are able to pay one then the other will be affected the following month if you don't have the funds. I'm not sure how quickly they'll stop the use of the sim but a discussion with CS will help in your situation.
I'll have to be blunt with you, they will probably take a stance of "well you committed so pay up".
The forum advice is to try a couple of these:
a) sell the phones, keep the sim buy a cheap phone to get you through this difficult time. Use the proceeds to either finish the contract or in your case, continue to pay the tariffs but at some point, you will need to pay till the end of your contract.
b) if you're at the midway point of the contract, reduce it though this is normally around £5 lower.
c) see if someone else close to you is willing to help out with taking on the payments for a short while. Important to note: if they fail to pay then you are still liable for all bills.
If you do sell a phone, you can use the proceeds to pay off a contract completely in one lump sum. This will be in the order of: remaining months x current tariff.
