Hi Szymon and welcome, you have come to the right place.
It looks like you have already thought about this a bit but I would suggest that you start about 2-3 months later, which ever way you head it will be really hot in June/July/ August.

It is possible in this time but your schedule is a bit rushed and allows no time for delays and stopping at places you really want to see more of, and there will be a lot of those en-route.

If 12 months is your limit, plan your route but be open to the idea of staying longer somewhere you like and shipping your bikes home from where you end up or skipping some of it, this trip should be about getting away from the rush of everyday life, not imposing more stress on yourself.

If taking one bike gives you more money for a longer trip then think about it, I think if two people are travelling together they should both be using the same type of bike but others disagree with this.

The idea of yacht hiking sounds good if you can get it, but it will add to your travelling time, friends found the cheapest way to get from South Africa to South America was to actually take a cruise where the bikes went as luggage, although this was some time ago and things might have changed since then.

The last person that I spoke to who had planned so much in such a short time told me there was no way they could extend it, their trip last 4 years in the end!

I hope you find some of this helpful.
Mark