Constantly keep two for loops running within a while loop?

For what you want, you should use coroutines.

Coroutines essentially eradicate any yeilds from the code

There are two ways you could do this.

1) Use coroutine.wrap, which will create a callable thread.

2) User coroutine.create to create a coroutine, then coroutine.resume to run it.

Here's an example of both..

coroutine.wrap;

spawn(function()
    while char do
        p1, p2 = char.Torso.Position, floater.Position
        local dist = (p1 - p2).magnitude
        coroutine.wrap(function()
            for i = 0, math.pi * 100, 0.05 do -- numerical
                weld.C0 = CFrame.new(3.5 * math.sin(i), math.sin(tick()) * 1.5, 2.5 * math.cos(i))
                weld.C1 = CFrame.Angles(math.cos(i), math.cos(i), 0)
                s.RunService.RenderStepped:wait()
            end
        end)()
        for i, v in pairs(rainbow) do -- table
            e.BrickColor = v
        end
    end
end)

coroutine.resume / coroutine.create

spawn(function()
    while char do
        p1, p2 = char.Torso.Position, floater.Position
        local dist = (p1 - p2).magnitude
        coroutine.resume(coroutine.create(function()
            for i = 0, math.pi * 100, 0.05 do -- numerical
                weld.C0 = CFrame.new(3.5 * math.sin(i), math.sin(tick()) * 1.5, 2.5 * math.cos(i))
                weld.C1 = CFrame.Angles(math.cos(i), math.cos(i), 0)
                s.RunService.RenderStepped:wait()
            end
        end))
        for i, v in pairs(rainbow) do -- table
            e.BrickColor = v
        end
    end
end)