Cloning a tube of yeast?

Normally I put one tube of WL yeast into a one-quart starter, made with one cup of DME.

I was thinking, could I double the size of the starter (two quarts, two cups of DME), pitch one tube of the WL yeast, and double the amount of yeast, perhaps by giving it a bit of extra time (instead of stepping it up)?

I have a magnetic stir plate.

Or, if I want to make the equivalent of two starters at once, do I need to pitch two tubes of yeast?

Thanks in advance.

I think that should work.
If there's enough oxygen, twice the wort should give twice the amount of yeast.
I'm sure there are a lot more variables.
This is a good calculator for yeast pitching:
Mr Malty Pitching Rate Calculator

I would use one pack of yeast and give it an extra day on the stir plate.

I have spit the packs 4 or 5 ways for yeasts that I really like. Its easier than washing, and if your sanitation is good, then you could do this almost indefinitely. Incidentally, another good technique is to ferment a small starter almost to attentuation (in a big bottle). Cold crash it, poor off the spent "growth media" (=beer) and then add fresh wort at 70 F. Stir it, wait a couple of hours and voila, you have a large volume starter without actually needing to add a large volume. As an additional bonus, you can choose to drain off the spent growth media and add fresh wort several days after the original the original growth phase is done. Just keep it refrigerated.