Call up Saunders in Panama City and ask 'em for what Karl used to run
Its a Fleetguard product and the dose is on the bottle. You flush the system out well (run the Penray online cleaner for a month before you change it over if you've been lax in this in the past) and then refill with distilled water plus the appropriate dose.
The strips come from the same people - basically the deal is that you need to buy the strips from the same manufacturer as the chemicals.
The strip will show no freeze protection (duh!) but the nitrate level is the important one - that's the inhibitors. Once you fill with the recommended dose you check it - should be ok. Then every couple of months dip to check the nitrate levels, and if low, add a bit more of the SCAs until its ok again. Enough SCAs to do both engines is something like $30 - dirt cheap compared to buying 10 gallons of glycol! Of course you still have the cost of the distilled water.....
This path is cheap enough that you can dump the water + SCAs when you go to lay up in the winter (if you do) and refill with the glycol mix for storage. Just run the engine for a few minutes to distribute the mixture before you pull the boat. If you're in a year-round "no freeze" environment then I'd change the water + inhibitor mixture every 2 years (same for coolant), just because cooling system problems are no fun and get expensive.
The other thing about water + SCAs is that it won't drop out and make that gummy mess in the heat exchanger that necessitates an ultrasonic or hottank bath.
The final advantage with water + inhibitors is that they're (mostly) non-toxic, so there's not the concern that exists with an accidental release of glycol, which is highly poisonous to marine life. The water + inhibitor mix can go down the drain (nitrates are fertilizers, so intentionally pumping it overboard is not cool, but its not the disaster that an actual POLLUTANT is.)
Note that if you have modern engines this route is VERBOTEN. Its good with Detroits because they run 180 degree thermostats and should really never run over 185F, plus Detroits are almost always under-cooled in terms of their maximum capacity, so the extra cooling you get from running water is a very good thing. Modern motors tend to run 195s or 205s and 15psi caps instead of 7, and they WILL boil over if you run water + SCAs - those engines NEED coolant.