Just update to 4.3.x on your iphone4. This will give you the 'Wifi Personal Hotspots' feature which will allow you to turn your iphone into a wireless hotspot which you can then connect to with your ipad's wifi.
If your iphone is Jailbroken then you can install an app called 'TetherMe', and also an app called 'TetherMe APN Editing'.
These apps allow you to edit the APN settings used when using 'Personal Hotspots' so your data provider won't be able to detect it as internet tethering, and therefore wont charge you.
o2 expected me to pay for two 3G contracts, one for my iphone4 and one for my ipad2 which I think is ridiculous, if i'm paying for 1GB a month then what does it matter to them what device i'm downloading it from. Imagine if broadband suppliers demanded you have separate broadband accounts for each computer/device in your home.
So there you go, no need to swap SIM cards, no need for a 3G on your ipad2 and no extra charges!