Do your Sen/RT player often get into conflict regarding a deal or commerce test?

In my games, both the Sen and RT have extremely high commerce skills and have been annoyed at each other as they both like to do the commerce thing and negotiations.

They didn't get to the point my AdMech and AM got the last time (where the AdMech would call the AM a "cogitator-damaged servitor" in front of his troops, who would then order him to be locked inside a chimera). But I felt some attrition between the players nevertheless.

Any suggestions for building a better synergy between my Sen and RT, specially on commerce/acquisition roleplay?

I've already told them that the next time they start bickering about the deal in front of the customer (as it happened last time with an Elder Tactician in Zayth), I will consider it an automatic fail on the commerce test, regardless of their skills.

And for the rest, I think they are fine (Sen handles spy networks alone, RT takes the macro decisions for the Dynasty).

Seems like a problem with the players. I suggest having them talk it out OOC.

how about parallel commerce deals, for instance the RT negotiating with some kind of big wigs and the Seneschal with a key underling or an other bigwig. After all, the best way to manage the RT must honorable tradition - that is, selling the same thing at least twice to different buyers- is doing this simultaneously ?

You could suggest that they both roll, and then average out the number of successes. That minimizes the wild crazy swings on dice, but it also minimizes the chance of an unlucky roll screwing things up.

Magellan is right in that your players should work this out themselves, since even though the Rogue Trader (theoretically) has the final say in everything, all players are equally subservient before a GM. If you allow them to work as a team then they can still bicker when one person does better than the other, but hopefully the tone can be kept positive and they'll be working towards a common goal.

Or use assisted rolls as per the book, have them take turns in being the primary and the assist.

The players ought to work it out I think. While both are great at Commerce, one of them will end up being better than the other one and thus the primary commerce person. In the game I'm in where I'm the RT and my sister is the Senechal, we both started out near equal in Commerce. But we hashed it out and decided I would go Commerce while she delved in to other things like knowledges and molesting xenos.

Otherwise I like both Erathia and Kasatka's suggestions.

Only ever had a Seneschal in one game, and the one that lasted the least number of sessions at that.

I'd think that where both characters had similar skill abilities, the Seneschal would take the test, aided by the RT, along with the RT's +10 bonus, unless roleplay hierarchy dictated otherwise. Doesn't seem like that big a decision.