Hi

Just installed and tested using my Aus and Thai sim card.

option to manually switch between sims or auto switch on a time lapse basis or auto operate both at the same time 

I’d show you the purchase details but ebay is blocked at the office. 

this was the ebay product decription Dual SIM Card Adapter + Back Case For APPLE iPhone 4 4G
I paid aud$14.00 + $7 postage from seller in Hong kong (seller name: heronfengstore2009)

They don’t have them in Bangkok yet 

	Description: indispensable to businessman! Dual SIM Adaptor + Protective Case. Enable you to enjoy 2 numbers in 1 mobile.   webpage details of the 2 sim card http://www.szfeifan.com/goods.php?id=93   there is an option for a 3 sim card model as well as other accessories    NB:  The accessory commes with a
	The world's first and unique design of two numbers in one iPhone 4 brings great convenience to your life & work, indispensable to businessman! Crafted from a super tough PC skin with tactile ergonomic design, this stylish and lightweight case doesn't just look after your iPhone 4; it looks after your hand as well.

You might be thinking - why not set the line to forward when in the other country
you can set the call to forward ie., from one number to the other but that does not work on SMS or the iphone Whatapps a net based sms

about mobile phone costs
there is a lot of hidden costs - be warned - view the Communication section of the forum to see the other articles - hope it saves you time or money

Thai sim card costs - huge costs savings
I recorded info re the savings of using a Thai sim card on arrival in Thailand. The info was placed int the faq section.  Aussie exec Tours - FAQ - Communicatiosn systems 

Feedback in this forum appreciated to share with other travelers.