I wrote a bit about this on the old forum - I don't have a link I'm afraid, but I'll summarise.

The a'dam is indeed a link between two channelers, but it's not like a normal circle. The a'dam forms the link and acts as a sort of compulsion device, but although the sul'dam is in control of the results of the circle, she is not actually leading it. The damane is. The Power flows through her and her alone - we've never seen a Power-nimbus around a linked sul'dam. The damane weaves the flows and embraces and releases the source, and it's the damane's Talents and abilities and restrictions - including Oaths - that matter.

I know it looks like the sul'dam is leading, and we did see Tuon use Joline's channeling as though it was her own, but I believe this is an illusion created by the fact that the sul'dam has utter and complete control over the damane's channeling. The sul'dam can have her weave whatever she wants, without ever having to command it. It is the damane's channeling, but the a'dam compels so efficiently that the damane need never be consulted at all.

In some ways the 'linking' aspect of the a'dam is almost secondary; it's almost entirely passive, and I don't really understand how some aspects of the link can work - the sharing of emotions, ability to inflict pain - despite the circle never completing and the sul'dam never embracing the Power herself.

But in answer to your question, no, a sul'dam cannot compel an Aes Sedai damane to break her Oaths, any more than they could have her weave a firestorm if she could barely channel Fire at all. If the damane is not capable of something, no amount of compulsion can make her do it.