Is the radiation above or below the main? If below, you have to use two monoflos and space them apart the length of the radiation.
The idea is to create pressure drop between the tees to get flow to the radiation. The greater the spacing, the greater the pressure drop, the more flow according to theory. On downfeed, more pressure drop is required to overcome the natural buoyancy of the hot water, as compared to the colder water in the radiation below.
On above the main radiation, the distance required is not as great. But you have to overcome the friction loss of the risers and radiation. The further away the radiation is from the main, the greater the distance required between the tees. Sometimes the radiation is far enough away to require two monoflo tees on upfeed radiation.
So...it all depends. Can you describe your situation in more detail?
Best regards, Pat
You will not "stop the flow", if you install them close but, it does pay to know how and why your doing it a particular way other than another. What are you trying to heat? Is it a CI rad, baseboard, coil, ect.? Knowing the load and flow needed would be where I would begin.