Say that I had two vacuum cleaner motors/turbines motor. If I connect the inlet of one to the outlet of the other, in theory I should obtain a better suction/flow at the other end?This flow/pressure at the outlet end of the final motor would be greater than only running one of them,   provided I maintain the same hose length.

Is this correct?

Depends on what kind of impeller design is present.

When I emptied this tank of its sand I hook up two shop vac on suction end,put one T facing down so that vac would fill first

By-pass, Tangential.

No

A vacuum can only suck in what it can blow out. The intake must equal the exhaust and visa versa. If both vacs are the same and have equal suction, then vacuum 2 can only suck in what vacuum 1 is blowing out, no more.

Lower pressure drop across each vacuum impeller = lower power draw on motor = higher motor rpm. Higher rpm & lower pressure drop across the impeller = very slightly higher volume flow = more pressure drop at the hose nozzle

If the vacuums are both the same putting 2 in series SHOULD provide a slight advantage, but it won't be much.

Paralleling them may actually provide as much or more advantage for the same hose-size because the airflow will increase significantly yielding a significantly lower pressure at the nozzle.