I'm not sure, but... I may be misunderstanding you.

It's always necessary to install Windows first as it will overwrite
the Boot Record and disable any other Operating System. Ubuntu isn't
so rude! You are asked in the Ubuntu setup whether you want to install
it to dual boot or take over the whole hard drive. If the former it
creates a boot menu which allows you to choose each time which you
want to start. When Ubuntu is installed it will create or modify grub
for you, other methods I wouldn't want to guarantee.