Again, in the barn with the odd angles and layout.

The laundry room and shower share a common wet well.
The laundry room is 3 feet higher than the bath, and I can slip a 2 inch drain line from the laundry and just about hit the trap for the tub.

Can I connect the two drain lines into a single 2 inch line?

The order would be tub drain, trap connection to laundry drain line.

1) I am not under a code obligation here.
2) there will not be a time when laundry is being done and a shower being taken.
3) if I had to, I could probably slip in a 3 inch line after the connection.

Thanks.

I do not know what you mean when you say you can "slip" these lines.

Slip = install.
I would use 3" if at all possible and tie in downstream of the shower trap as far as feasible. It will not be code compliant but it will work.

you realize that in some areas they require a 3 inch drain for the washer?
so at very least use a 3 x 2 x 2 "Y" for joining them together.

Please post some pictures as it is difficult to visualize your layout. The washing machine and tub sharing a wet vent is a code violation. These two fixtures need to be vented separately. Section 912.1 of the International Plumbing Code under sub-section 912.1 Only fixtures within bathrooms are permitted to be wet vented. Any fixtures other than those in the bathroom must discharge downstream from the bathroom group.

As far as your washing machine goes... Section 406.2 of the International Plumbing Code states, " The waste from an automatic clothes washer shall discharge through an air break into a standpipe in accordance with Section 802.4 or into a laundry sink. The trap and fixture drain shall not be less than 2 inches in diameter.

Here is the kicker section 406.2 goes on to say,"the fixture drain shall connect to a 3-inch or larger diameter fixture branch or stack". This help eliminate trap siphonage and sudsing. In other words the pipe under the floor and up to the fixture would need to be 3". You would need a 3"X2" san tee to connect your washing machine standpipe and p-trap to the 3" fixture branch.

OK...thanks.

I can get the 3 inch line in and connect the tub and the laundry to that, and connect the 3 inch line to the 4 inch main sewage line.

Will I need to add venting for the laundry, or can I rely on the 3 inch main stack?

I probably should have mentioned...I am not under any code requirements in this instance, but don't want to have any issues.