Tankless Water Heaters in Series
Won't work.  Either the point-of-use heater is big enough to heat the water, or it isn't.

If it is big enough, you don't need the main heater at all.
If it isn't big enough, the water will come out tepid for the first minute.

Both of these scenarios are losers.  You're spending a lot of gas to heat water that'll just go down the drain, because nobody's going to step into that shower if the water is coming out 80F.
You are better off rearranging the furniture so that the 70,000 BTU water heater is adequate for that end of the house, and having it be the primary water heater.
Last time I did a calc like this, it was with a 13KW heater that would barely make a shower work with a 1.2-1.4 GPM flow on the shower head.  Let's convert your 70,000 BTU/hr into KW.
1000 watts provides 3400 BTU/hr.  So it looks like your 70,000 BTU/hr heater is equivalent to a 20.5 kilowatt (85 amps) electric heater.  That's doable; put a low-flow showerhead on there and you should have some operating margin.  Simply have that heater supply that end of the house (the closer to the shower, the better - for response time) and have the new heater serve the new wing.
Keep in mind that if you draw water at a faster rate than the heater can keep up with, the heater heats the water less.  That's when the water goes tepid and users go &quot;WHAT THE HECK on-demand heaters!&quot; and they wrongly conclude that on-demand's suck and get rid of em. That's the wrong move.  When the water goes tepid, reduce flow and it will heat up again.  Of course good luck teaching that to users; so I advise putting a gate valve on the inlet of the water heater, and adjust it to impede flow, to make out-drawing the heater impossible :)