I am currently designing a network, and there are 2 fibers between 2 switches. Can I use a 40Gbit QSFP+ module to achieve this speed or is 10Gbit SFP the best possible without adding more fibers?
QSFP+ is just the transceiver interface. Medium compatibility and reach is defined by the PHY used by the fitted transceiver.
While the short-range 40GBASE-SR4 PHY requires four separate multi-mode fibers per direction (OM3 or OM4), 40GBASE-LR4 (CWDM) or -FR (single-lane) use only one fiber per direction - both require single-mode fiber, however. Both will work on short MMF runs as well - unspecified though.
Beyond the official IEEE 802.3 PHYs there's a range of other types available - many from third-party vendors.