If I sling the 40 and the dive goes good, i would not have used the 40 at all. only 1 refill.... if it doesn't go good and i do use the 40 i will only use a very small amount at the very end of the dive to get back to the boat. The 40 can be reused with out having to be refilled.
I will echo the point that Thalassamania is making on this one. Plan to use your AL 80 for two dives. Sling your steel 40 pony, as a reserve, just in case you need it on the second dive. BUT, if you use it, at all, top it off before you use it again. Set a minimum for yourself, even if it costs a little more (which isn't a big deal, you are already getting two dives out of an AL80). The best way to avoid having to top it off - plan to surface after the second dive before you have to use your pony, even if that means every once in a while, cutting your second dive from 60 to 50 minutes.
Sure, you could also use, two smaller tanks (e.g. 63s). But, why? Then you are filling two tanks after every two dives. If you use your 80, stick to the plan of not dipping into your 40 unless you have to (or make a conscious decision to use it to extned your second dive), generally you will be filling only one tank after every 2 dives.
i can probably get through at least 3 or 4 of these before the pony needs a refill. still 1 ...fill (im 90% sure i wont need the pony)
And, this is the slippery slope that Thal is encouraging you to avoid. I would encourage you to avoid it as well. What is the cut-off pressure for refilling your pony? Is it 2000, 1500? Would you 'probably' be OK to go in for your second dive, with 1800 in your AL80, and 1500 in your 40? Yes. Is it a good habit to get into? Probably not. In reality, it is a matter of good dive planning discipline to avoid getting into that situation. The 40 can be there for a) a reserve, or b) extension of your dive. It is OK (although not necessarily recommnedable) to use it as the second option. But, set a minimum threshold for that use, which is that it is FULL when you start the dive.