There are one or two 3rd party boot managers that claim they can do
this. Google "boot manager" and check specs.
There are also one or two virtual machine (VM) systems that do it.
No Windows can do this natively. Some other OSs may be able to.
Be careful. This capability is often accompanied by a dangerous
vulnerability to intersystem corruption, especially with boot managers
(I believe - I'm not sure how they actually do what they claim to do). A
well-written VM system will protect against this better, often using
hardware to allocate and handle virtual address spaces and myriad other
duties.
Since your systems are already installed, you may be constrained in your
choice of which boot manager or VM to use. I'm not certain of that
either. Uncertainty is a big part of my life, and often entertaining.
Good luck. It'd be nice if you'd post back with a success story.
