ON a 4-stroke cycle engine, it takes TWO revolutions of the crankshaft to complete a combustion cycle. So a cylinder is firing every 180 degrees of crankshaft rotation. Let's look at what happens on an engine with a 1-3-4-2 firing order.
When #1 cylinder moves from TDC to BDC on the intake stroke, the intake valve is open. As the piston passed BDC and moves into the compression stroke, the #1 intake valve closes. At the same time, #3 cylinder has the intake valve opening on its intake stroke. When #1 fires and begins its travel to BDC on the power stroke, #3 is moving up to TDC on compression stroke...and #4 intake valve is opening on intake stroke. As #1 exhaust valve opens and #1 cylinder moves from BDC to TDC on the exhaust stroke, #3 is on its way down to BDC on the power stroke...while #4 is on its way to TDC on the compression stroke...and #2 is on its way to BDC on the intake stroke.
So tell me...at what point do two adjacent cylinders BOTH have their intake valves open?
Not trying to be a smarta**..just trying to understand what you're saying, as it relates to a 4-stroke engine. [On a 2-stroke engine, the intake valves on #1 and #4 would be open at the same time, and the intake valves on #3 and #2 would be open at the same time. But 2-stroke 4-cylinders are extremely uncommon here in the US...so the assumption is that you were talking about a much more common 4-stroke engine.]