My wife is a Thai & US citizen and we want to live in Thailand with occasional visits to the US.  The Thai embassy here in the US told me that having the 2 citizenships is technically against Thai law but that no one would say anything about it.  He also said they want my wife to use her US Passport when entering Thailand but this cannot be possible since we want to live there and I dont her to be making Visa runs for being a US citizen.  She must use her Thai passport when entering or leaving Thailand.  She must use her US passport when entering or leaving the US.  My question is, will any of the immigration officials from either country closely pay attention to the stamps in the passport to actually see where she has been and for how long?  I have never seen anybody really study them before as long as you have more than a few stamps in there.  Any ideas???

You are right, the consulate are wrong. She use her US Passport to enter and exit US, and uses the Thai passport while entering and exiting Thailand.

The immigration does not care about the foreign stamps.

At airport check in she might be required to show the other passport as well, as airlines want to be sure that she have visa or is a citizen of that country.

If you search the forum for dual citizenship for the last year, you will find a couple of threads covering this topics.

Will add that there are few laws on this subject so it is mostly a grey area and civil servants don't like that.  The concern is that you are now subject to two sets of laws and 'the other' country is restricted in what they can do to help you when you travel under another passport.  So if you are subject to military service that could be an issue.  If you enter on the passport of the home country and end up in jail I expect that you should expect to remain in that countries jails.  But for most people it is not a problem.

I believe that your Primary Citizenship always takes precident as a dual national.

That is, if a Thai national becomes a US citizen (without renouncing) then if, and only if, a clash comes as to your nationalities law's you would be counted a Thai citizen. For example: If this person was arrested in Thailand, the US Embassy would not be interested as (s)he is a Thai Citizen primarily and a US citizen secondly; conversely, if arrested in the US, as long as (s)he had not renounced Thai citizenship, then the Thai Embassy could be willing to help.

If there is no clash of law (or interests) then all is well.

I believe that your Primary Citizenship always takes precident

I don't believe governments can even agree on what "primary" is.  You could be born with two citizenships and many are.  Others are citizens of provinces and such within a state.  I still think it is a huge 'gray' area.  The only recent change is most countries now do not try to prevent dual citizenship as much as they used to.