Tattoo collaboration & tip etiquette question

[Harlot]

I'm going to be getting a tattoo that's a collaboration with 2 artists. It sounds like one will be spending more time tattooing than the other because of the way it's being designed. How should I handle tipping each one? I've never had a tattoo done like this and I don't want to offend anybody.

[Michaelshane]

Why don't you just ask them?

[SStu]

proportionally by hours done?

[rufio]

just tip after each session.

[Hands On]

now that all that's settled, who are the artists and what are you getting?!.. (if you don't mind sharing). =)

[Harlot]

It's being done in 1 session. I'm getting Einstein done by Geary Morrill (Unkindness Art) and Mike Riina (Eclectic Art Tattoo Gallery). They have completely different styles so this should be fun.