So, there are some dommes (sic) here and there who at times bring in an extra guy who is partly unexpected from the point of view the first/main/sub.
i will comment as a sub. First of all you are making the assumption that all Dommes follow the same level of Professional Conduct and sub Safety or that there is a stand set of rules or guidelines.
Remember that set of questions you answered before you met with a Mistress. Those were your limits, hard, flexible and open.
Limits may evolve over time over time if you develop a longer term relationship with a Mistress, however it would be unsafe and unprofessional for a Domme to simply introduce another male to a scene without a lot of discussion, training and play leading up to that situation.
There would be no partly unexpected cock walking into a scene. A Professional Mistress would have through prior sessions created the sub's agreement in principle long before this "introduction" occurred.
If and when the first sub does not go along with forced bi, have you who are dommes(sic) ever let him "escape" but with, say cum on his face, but no bi itself?
It is clear that you don't understand what Safe, Sane and Consensual means. If you are not with a Dominatrix who gives you safe words, you are putting yourself in danger.
Stop means Stop
Or, if you are a sub, has that happened to you at times?
see above