This would work out that the cross-section of 1/4" to 3/8" pipe would yield sufficient refrigerant speed. But when splitting our 'pipe' in two parallel parts, volume flow is halved, but refrigerant speed stays the same.
If we split the flow between two parallel circuits whose cross sectional areas add up to the same cross sectional area as the original, then both total volume and velocity remain the same.

If you mean vapour velocity: there are rules of thumb which indicate minimal refrigerant speed to ensure decent oil transport. Of course these values differ for rising lines and horizontal lines.
If cross sectional areas are equal to suction line cross sectional area throughout, then velocity should remain constant throughout.