It depends on the insurance company, the more risk with a precondition the more they are likely to exclude it, although they normally exclude it for a period of time.
the FET is used for a foreign transfer, so money coming into your thai bank from your bank account abroad, I would suggest you contact the land office if your paying another way, especially if its a resale property.
The FET is form which cost a few hundred Baht and you can get from the bank you tranfer the money too, I'm pretty certain it's needed at the land office as prove of funds being transfered abroad.
They UK government want to see she has sufficient ties to Thailand that she will return, for example if she is working a letter from her employer, house in her name and some money in the bank, the actual amount is not so important as long she can demonstrate she will return to Thailand.
There is another way of looking at this, that is to have two numbers on one phone contract, which means you pay one bill for two numbers. You will numbers given two sims as outlined by the other replies. AIS and probably the other big suppliers can provide this.