I see, Thank you everyone for kind help and preventing me from burning down something.
I've done a bit more research on this and found out, that basically the charger just won't handle it.
2x18350 = 8.4 volts fresh from charge
1x18650 = 4.2 volts fresh from charge
Now I know that I probably should not do it, but, theoretically - if I use one 350 in 650 charger and fill blank space for example with tinfoil to make a connection. Would that work out? Sometimes I have "weird" ideas, but before doing anything like that, it's better to ask for opinion. Thanks once again.