Do you mean you are using the connections at the same time?
Or just that you use one or another at different times? I suspect the later...
I use multiple connections (but not at the same time) which creates a problem because the connections have different nameservers in /etc/resolv.conf. Rather than reconfiguring each time, make different copies of your "dsl-provider file" and run the whole thing with a script that copies the correct one in first:
cp /etc/pppoe/providerfile.copy1 /etc/pppoe/providerfile &&
pppoe-up (or whatever command you use)
Just call that script "connect1" and that's all you'll have to type next time. I don't use pppoe so I'm not familiar with how to launch it, but hopefully the logic here is clear: the destination file of the copy is the real name of the file that pppoe uses, the source file is one version of what it should look like.
Important: use the double ampersand (&&) after the cp command to make sure it completes successfully before you attempt to connect.