Yes. A couple of things to note here:
- Because the entire process of initiating an attack is considered to happen simultaneously, each character chosen as a stealth target is considered to be bypassed simultaneously. This means that there is no point in time where the character chosen by Stealthy Attacker #1 has been bypassed, but the character chosen by Stealthy Attacker #2 has not. (This should be evident in the fact that the same character can be chosen as the stealth target for two different attackers in the first place; if the bypass was not simultaneous, being targeted by #1 would make the character ineligible as a target for #2.)
- The upshot of this is that when two attackers with stealth choose the same stealth target, the defender's character has been successfully bypassed twice (at the same time), even though there is no practical game benefit in terms of Stealth effects on a character bypassed more than once.
- The triggering condition on Marya Seaworth is when a character is bypassed using stealth. As such, the fact that the character in this scenario is successfully bypassed twice (once each by two different attackers using stealth) creates two separate and independent triggering conditions for Marya's reaction.
So, since there are two separate and independent triggering conditions for Marya's reaction on a single character chosen as the stealth target for two different attackers in the same challenge, Marya's controller may trigger her ability a second time (in reaction to triggering condition #2) if their opponent cancels the use of her ability in reaction to triggering condition #1.