They can easily get both. Haaland won't be on that big of a salary.... maybe 175k to 200k a week. Messi will be free, so they'll only have to cover his salary, which won't be anywhere close to what it was with us. They can also sell certain players like Mahrez and Zinchenko, so that'll help towards getting Haaland and Messi. They could also try and get in Alaba, which will also be a free transfer. I doubt he'd want any more than 250k a week. If Aguero leaves, that'll free up another 250k a week..... but I think Aguero will stay another 2 years on a lower salary.
If they sell Mahrez for roughly 40m to 50m, that will cover Messi's signing-on fee and salary for the year (Mahrez is on roughly 150k a week). They could then sell Zinchenko for a decent price, so that could cover Alaba's signing-on fee and salary. The only thing they have to worry about is Haaland, but I think they could manage it. Even if they don't get Haaland in the summer, he has a release clause of 70m euros in 2022. So, realistically, they would only have to spend money on Haaland.
It's not impossible for them to get both Messi and Haaland. It wouldn't affect them that much in regards to the FFP.... especially if they sell certain players.