That's strange. Don't your providers offer a secondary SIM card on the same contract? That's what I'm using: 33 € with 2 GB for my smartphone's SIM card. And now an additional 4 € a month for a secondary SIM card for the same number. I'll just buy/insert that secondary SIM card into the tablet and that's it. Just 4 € more.
When I was with o2 it was only business accounts that had that option, but they were only willing to let me have a second phone sim, not one for a tablet. Most carriers will nuke your SIM if you try using it in a tablet. I guess because they expect much higher usage with a tablet than phone. As it stands though, having a tablet SIM makes no financial sense to the vast majority.