If each appliance is served by a separate run from the meter (all branching is at the meter), then you can just consider each appliance separately. If one of those runs is 40' of 3/4" and 40' of 1", the order doesn't matter. You can determine the maximum allowable demand at the end of that run as follows (assuming you have the correct table for your pipe type and regulator pressure):

You have 80'. If it were all 3/4" it could supply 117 BTU/hr. If it were all 1" it could supply 220 BTU/hr. Since it's a mix, it will be somewhere in between.

To actually figure out the allowable demand at the end of the run, you'd need to do something like this: Guess that it could be 150 BTU/hr. Per the table, that would be fine with 50' of 3/4" (151 BTU/hr on the table), or with 150' of 1" (157 BTU/hr on the table).

So 40' of 3/4" uses up 40/50 = 80% of the allowable pressure drop. And 40' of 1" uses up 40/150 = 27% of the allowable pressure drop. So that makes 107%, meaning 150 BTU/hr is probably a bit too high. Except that 157 > 150, and so 27% errs on the high side.

The answer is going to be 145 - 150 BTU/hr.

Cheers, Wayne