So 2M is 200mls per liter .
If you had 500lms of 1M it would contain 50mls HCl and 450mls of water
Therefore you would need to add 150mls of HCl and 350mls of water to make it up to a liter .
If you had 750 mls of 1M it would contain 75mls HCl and 675mls of water .
Therefore you need to add 125 mls HCl and125mls of water .
If you had 800mls of 1M it would contain 80mls of HCl and 720mls water
Therefore you need to add 120 mls HCl and 80mls of water .
If you had 850mls of 1M it would contain 85mls of HCl and 765mls water
Therefore you need to add 115 mls HCl and 35mls of water .
If you had 875mls of 1M it would contain 87.5mls of HCl and 787.5mls water
Therefore you need to add 112.5 mls HCl and 12.5mls of water .
If you had 885mls of 1M it would contain 88.5mls of HCl and 796.5mls water
Therefore you need to add 111.5 mls HCl and 3.5mls of water .
If you had 890mls of 1M it would contain 89.0mls of HCl and 801mls water
Therefore you need to add 111 mls HCl and -1mls of water .
So add 111mls of 32% HCl to 890mls of 1M HCl should bring it up to 1 litre of 2M
My math brains not working either . I'm sure there's an easy formula to work this out
Edit:
Actually , the more I think about it , it is very similar problem that Vinters have when they need to Fortify wine by adding a stronger strength alcohol .
I believe that it's a mathematical procedure called a Parsons square which is used to derive the answer . There are online calculators that work it out .Suely there would be one for this type of application too .
Is it 
time?