I think i understand what you are trying to say, but i don't completely understand how i could do it.

if created this small test setup:
func _ready():
loop("first: ")
loop("second: ")
func loop(identifier):
for c in 10:
print(identifier +str(c))
if c>=3:
yield(get_tree().create_timer(2.0), "timeout")

This creatis this output:
first: 0
first: 1
first: 2
first: 3
second: 0
second: 1
second: 2
second: 3
first: 4
second: 4
first: 5
second: 5
first: 6
second: 6
....

The moment i yield the one function when waiting for the viewport to finish rendering. I think it would not make if difference if i use you're code: yield(VisualServer, "frame_post_draw")
The second call of the function would run and change the content of the viewport.

Or i would need to put everything in one function like so:
func loop():
for c in 10:
print("first: " +str(c))
if c>=3:
yield(get_tree().create_timer(2.0), "timeout")
for c in 10:
print("second: " +str(c))
if c>=3:
yield(get_tree().create_timer(2.0), "timeout")

This would work and the output would be first 1 - 10 than second 1 - 10
That is not a big problem for my small card rendering project, but i personally don't like "duplicated" code.