If you create a multi-boot by adding entries to the BCD, Microsoft will display a boot menu. If you turn on "skip menu", there's no way to circumvent that during boot. You would need to turn skip off before rebooting, then turn it on again afterward.
Not a lot of point doing it that way.
If the OSs are on different devices it's much easier not to create a multi-boot in the first place.
Just let the BIOS go to the device of choice by default, and override it during boot with F8 to select an alternate boot device.
If your OSs share a device, then you're stuck with a multi-boot.
The simplest way to manage that to approximate to your request, is to set the timeout value to (1).
The boot will choose your default OS with a mere flicker of a boot menu choice.
Should you wish to choose an alternate, begin tapping the down-arrow key during boot.
When the boot menu is displayed, the down key will have stopped the timeout clock, and you'll have as much time as you need to select an OS.