Combining partailly used MSR isobutane gas canisters?

Note that in addition to a double-ended adaptor, you’ll need to have the canisters at different temperatures so that liquid can move from the upper to the lower canister.  If you’ve used both canisters only in liquid-feed mode, their vapor pressure at any given temperature will still be similar and you won’t need a huge temperature difference.  But if you’ve been using them both in vapor-feed mode, the fuller canister will be more propane-rich and higher pressure than the emptier canister so you’d need more of a pressure difference.  (Although, I suppose you could go the other way, for instance empty 120 grams from a 220-gram canister into a 220-gram canister that had only 50 grams remaining.  Then the pressure difference would be in your favor and would stay that way as you did a liquid transfer).

But you need a check valve or a closable valve so you don’t vent one canister as you are still connected the other (and likewise when you disconnect the first one).

And THIS IS IMPORTANT: you need accurate empty weights on both canisters and never try to combine two amounts to be more than the original quantity of fuel.  Yes, those canisters are rated to a decently high temperature by DOT, but that presumes they weren’t over-filled.  As long as there is head space in the canister, it is at the vapor pressure of the fuel mix inside.  But that hydrocarbon fuel inside expands much more than metal and if it completely fills the canister, there is no limit to the pressure a liquid-filled container can go to.  Well, there is a limit and it involves the canister rupturing and spraying fuel all over, hopefully in your car trunk and not, say, in a tent while attached to your running stove!

Might have just found a solution:

https://www.amazon.com/Gas-R1-Cartridge-Exchanger-Duralumin/dp/B00TXZ4JUA/ref=pd_sim_468_1?ie=UTF8&psc=1&refRID=4SEG1TVVRBJYG6HX2HBY

All of which is to say that is a varsity-level stuff there.  All to salvage a few ounces of fuel.

Weigh your canisters when new and mark their full weight with a laundry marker on the bottom.  When you’ve emptied one, get an empty weight and mark all the others you have of that type.  Now you can always check precisely how much you have remaining.

There are actually times you WANT a less-than-full canister.  Say on a one- tr two-night solo trip.  You don’t need 220 grams, or even 110 grams of fuel so bring one that is a bit over what you need.

How much do you need?  Weigh your canisters before and after every trip and you’ll quickly learn how much 1-, 2- 3-person trips need per day in summer and in winter.

And when a canister has even less than a one-night, solo trip?  Toss those in a box for testing stoves, boil times on pots, and for car camping.

First you must ensure that you do not over-fill a canister. Weigh an empty canister first. Weigh your two partially empty canisters, subtract the canister weight and make sure the combined weight of gas does not exceed the capacity of one canister.

When transferring the gas, put the fuller canister on top of the more empty canister. This is counter-intuative, but the fuller canister will have a greater vapour pressure (assuming you have used it with an upright stove). Puting the lower canister in the fridge beforehand also helps. Even so, it can take half an hour for the gas to transfer.

Do it outside, well away from any flame or other heat source! All at your own risk of course.