The data itself is free though, which is the most important for me. It is definitely better than having to go to a phone store and get a cheap sim.
EG: when in the UK calling the US costs 56.2p minute, which is a complete ripoff as it should be cheaper than calling a UK mobile (virtually nil cost to do these calls). However, when actually in the US it's 15.6p. Same price differential for SMS.
Sending a text from the UK to a (for example) Spanish number incurs a cost outside of your allowance. So that same text would also incur a cost when you are in Spain.
This works well for me (since I travel between the US/UK often)
What's the purpose of the two SIPs?
Texting with Google Voice is fine though.
The two sips are registered to the same installation of asterisk: The UK provider gives me a UK number and rates equivalent to a UK landline.
The US provider gives me a US number (which is registered with google voice) and rates equivalent to a US landline.