You are making this harder than you need too. You just need a hydraulic sequence valve. They are used with one way cylinders like most markers have on them. They have one port in and two out. There is a piston that moves back and forth to alternate which port has pressure. So every time you apply pressure the value will alternate the next cycle.
Cross makes them. They are not that high a price. $75-150. I am having trouble finding them on any of the on line catalogs. I get them through John Day company. The one in the first picture is on a JD 1780 ridgid planter. JD wants $475. It has a pressure relief you do not need.
The picture is one used on a JD ridge frame planter. If there are any farm equipment junk yards around you you should be able to find a used one. Many planters use them on the markers. Any that had the markers on an independent SCV used them.
The one in the first picture would have the supply hose go right in the front. The the two ports on the top would go to each marker cylinder. The one I use has the front port just like this one but the back ports come out the back side by side. It is a cheaper valve than this one. This one had a pressure relief built into it that you would not need.
The second picture is more like the ones I use. It does not show much detail thought. It is on a JD 1290 planter with hydraulic markers.
Like I say a common valve if you know what to look for.