Can you drill through the wall or would you try to reuse the hole?

Professionals will do load calculations for you so you can decide on the size of the equipment. Quality minisplits throttle well so oversizing is not as big a problem, but you want to get them as close as you reasonably can.

One thing to consider is that if you have large gatherings in the LR (or the BR for that matter) your installer should take that into careful consideration. With conventional ducted AC, the system reserve capacity for the whole dwelling can be tapped to absorb that body heat. Since there is no circulation with minis, that one unit has to handle it all.

Depending on what brand you choose, you can get 6000, 7500 or 9000 BTU in the smaller sizes assuming that you don't really need 12,000. Have you considered ceiling-mount and short-ducted units? That latter are available in both high and low static pressure versions from some companies. You might make one unit serve both rooms, but you lose the inherent zoning then. OTOH, you might have your bathroom and BR served by one unit and your LR and kitchen served by another. There are also indoor units that sit on the floor. A lot depends on your layout.

Questions: What kind of heat do you have now? You might consider heat pumps if the gas/electricity costs are right, heat pumps could save you money. They may not be useful in the coldest weather at your lattitude. Note that the Mitsubishi single units are more efficient and work at lower temps than their 2,3, and 4 combo equipment. I can't say about the others. Critical in a situation like this is where does the condensate go? I guess there must have been some provision for the in-wall unit.

I don't understand your choice in your first sentence. I can see two meanings. 1) You are choosing between having only one cooling unit in the LR as before and having two by adding an additional one in the BR. 2) You are planning on having two indoor cooling units and choosing between installing two 1:1 systems on the one hand, and a dual outdoor unit with two indoor units on the other. If your choice is the latter, you will likely save some space on the balcony if you have a dual unit. It seems like you are saying that you have a balcony by each room, however, so installation may be a lot easier if you install two separate units. We don't know enough about the layout to say if you can easily run the plumbing from both locations to the multi. Another advantage of two separate units is that you will have backup in the event that one fails.

I think that you will be a lot happier with the MS than the sleeve unit.

i can drill holes as needed.
i don't know enough yet to answer well the questions ionized is asking.
i'll try:
a ceiling mount minisplit is what other people in the same building are moving towards.
1 outdoor unit and 2 indoor cooling units uses less balcony space.
a heat pump is kinda useless and optional because it only works until it doesn't. Or...

Heat pumps are moving toward more efficient performance at lower temperatures all the time. The last time I looked at Mitsubishi's claims for their 1:1 residential units, they gave figures that indicate that they get about 60% of peak performance in heating at -14 F. Performance seems to drop off starting at about 14 F.

I can't say how efficient it will be by cost. I think that the US DOE has some calculators you can use plugging in the numbers for specific heat pumps and fuel costs. It probably won't take over all your heating in frosty Montreal, but it might be very useful since I don't think a heat pump costs much more than a straight AC unit. You do have to do something with the ice that forms on the outdoor unit. That could be a deal-breaker on a condo balcony ;-(