So would both modems plug into it and then one ethernet cable into my computer, I assume.
Yes.

Is it worth the extra money to do this?
In your case, I don't think so. I would check if there is a higher bandwidth DSL from your provider first. That would probably be cheaper than 2 separate DSL lines + a dual wan router.

Will it supposedly double my internet speed?
It should be better, but not quite double. There is some overhead when you do multiplexing.