There is a product being sold called "Magic SIM".
Basically it is a SIM card that has two slots and a small memory chip. After physically cutting up your two SIM cards and putting them into the slots you insert the whole thing into your phone.
If your phone supports the GSM STK (SIM Tool Kit) standard then you can switch back and forth between the two SIM cards.
However for your phone to actually support the standard the manufacturer had to have built it into the operating system as well as provide a tool to "talk" using the STK.
I currently see that the Android development tree has some Stk development going on. I downloaded a precompiled Stk.apk and installed it on my phone with the card. However it does not seem to work.
While I do see the SIM Toolkit menu icon and it does seem to read from the custom card I am unable to obtain a carrier signal from either of my SIM cards. When I scroll to any of the menu options such as "SIM 1" or "SIM 2" and press the trackball the phone simply gives me a rotating circle on the upper right saying it is busy. And at that point it hangs.
Here is a quick and dirty image of my phone as well as what the SIM menu looks like: