The outlets of two fans with 5Kpa pressure and different air volumes are connected to a common duct. Is it possible not to install check valves at the outlets of these two fans?

No. For reference, if one of them stops operating for some reason, it can cause the equipment to reverse direction, damage the bearings of the equipment, or lead to a drop in system pressure.

First, as a general rule, a check valve or some type of blocking device such as an 8-shaped blind plate must be installed, depending on the process requirements. Next, regarding this design: generally, two fans are connected in parallel, with one serving as a backup to the other. When one fan is operating, the other one is not in use. Therefore, a shutdown device should be installed to prevent the fan impeller that is stopped from rotating in the opposite direction due to the airflow coming from the fan that is currently operating. If both fans are in operation, there are two possible scenarios. 1. The two fans drive different equipment; in this case, the fans operate along two separate process lines. It is advisable to keep the air ducts separate and independent of each other. If it is necessary to combine them, this should be done at a distance apart, using larger main ducts for such combination. 2. The two fans drive the same equipment; in this situation, it is possible to consider combining the two fans into one unit from the design stage. Generally, when the total pressure head and flow rate remain unchanged, using one larger fan is less costly than using two smaller fans, and it also avoids interference between the two fans operating in parallel.

Just add a one-way wind shield.

Based on the way the pipes are installed, if the two pipes meet at a 45-degree angle and merge into one main pipe, then it is not necessary to install check valves. Taking maintenance considerations into account, valves should be installed at the inlet and outlet of both fans.