That kind of question is quite hard to explain, but I can help you with the processes you need to take:
1) Back up your current MBR (first do a backup of the original one with dd if=/dev/sd# of=mbr.bkp bs=512 count=1 Consider # the letter of your main drive with MBR, and will generate a file called mbr.bkp in your current directory.
2) Install grub into your MBR with your 1st Linux, consider it as your "main" one, because it will be responsable of maintaining "grub" configuration. Is a bad idea to have 2 grubs overriding theirselves again and again when you have custom configurations, by default, should be fine.
3) In case your grub doesn't detect your Windows, you will need to evaluate case by case what would be wrong, but the chainload +1 will be enough. If this step fail, you can rollback your mbr with dd if=mbr.bkp of=/dev/sd#
4) Install your second distribution, Linux installers may override your previous grub from step 2. In case everything is fine, you should see both distros and Windows without problem due grub2 scripts for other Linux and Windows.
In case doesn't work and you only see one distribution, you need to check what the command grub-mkconfig shows and check if the scripts are detecting your 2nd distro and Linux correctly. In case of debugging, for grub2 the script 10_linux do a proper analysis of your partitions and copies the grub configuration of that distro alongside your current one, 30_os-prober will detect Windows. Also, if you have real problems, you can create your own configuration at /etc/grub.d/* alongside the other scripts.
Just make sure to NOT be at UEFI because Windows would make some problems trying to make grub working correctly due UEFI sign keys by default (Microsoft ones).