You are looking for a flow divider. There are two types that I know of: proportional and gear or rotary. My experience with the proportional types has not been good. (They were probably mis-applied.) The gear type has saved our manufacturing plant many thousands of dollars in repairs over the last few years. They synchronize the motion of two or more cylinders so that they both (or all) move exactly the same amount. Neither moves without the other moving simultaneously, regardless of loading. I'm talking about an industrial heavy duty application. Our unit costs $835 each. The mobile hydraulics world seems to be an industry unto itself and I'm not very familiar with it, but I would think a similar device might be available for a mobile application. Anyway - flow divider, that's what you're looking for.