Ok, I know this may sound completely retarded, but please bear in mind that I only have a basic understanding of hydraulics. Here is my thought, can two or more hydraulic pumps be combined in a single circuit to increase flow, resulting in an increase in pressure?
FLOW and PRESSURE are related concepts, but you're not correctly understanding how they're related.
The driving force in hydraulics is obviously the pump, and like all pumps, it has a "characteristic curve" that defines its flow and pressure capabilities. What this boils down to is if you were to test the pump under a variety of operating conditions, you'd find that within its operating range, for any given flow, there's a corresponding pressure. Change the flow, and you change the pressure. As might be expected, higher flow means lower pressure and vice-versa. The comment about "within it operating range" deserves some further clarification. Two items: 1) Pumps always have a maximum operating pressure differential. So for example no matter how low you drop the flow, you won't get more pressure above its maximum value, and 2) Often there's some means of pressure regulating device in a hyrdraulic system, which effectively restrict the amount of pressure/flow variation that the pump is exposed to.
Given these comments, applying them to a made-up example, a typical pump might be sized to deliver 1gpm at 20,000psi. As you're putting the cylinder to the load (and so the pressure is less than 20,000), its flow will be greater than 1gpm. When it reaches 20,000, it will be 1 gpm. If load increases above 20,000, flow will be less than 1 gpm. (Note that most manufacturers rate their pumps at the maximum pressure differential. Don't expect to get more than this.)
Now on to your question. The answer depends on whether the pumps are arranged in series or in parallel. Pumps in parallel will deliver more flow but with the same pressure differential. Pumps in series will deliver nominally the same flow, but at higher pressure. Makes sense, huh? Put the output of the 1st pump (which is at 20,000 psi) into the input of the second pump, and it can then apply another 20,000 psi boost. Same flow, but more pressure. The pumps in parallel scenario is probably now pretty obvious, more flow, but at same pressure.
This is an idealized analysis, and the real world will be somewhat different from this, but close.
Another key element of this is to think chain and weak link. Specifically, if you're building a system with pumps in series, be careful that all the hydraulic hoses, fitting, etc., are rated for the combined pressure of the two systems, and that the "high side" pump can handle the total pressure. Yes, it's only being asked to do its normal thing (provide a 20,000 psi boost), but because of the elevated starting point, the pressure inside the pump is 40,000 psi (using this example), and it may not be able to handle it.
