Many FPGA-based accelerators are constrained by the available resources and multi-FPGA solutions can be necessary for building more capable systems. Available PCIe solutions provide only FPGA-to-Host communication. In this paper we present JetStream, an open-source1 modular PCIe 3 library, supporting not only fast FPGA-to-Host communication, but also allowing direct FPGA-to-FPGA communication which fully bypasses the memory subsystem. The direct mode saves memory bandwidth for multicast modes and permits to connect multiple FPGAs in various software defined topologies. We show the benefits of JetStream with a large FIR filter spanning four FPGA boards, achieving throughputs of up to 7.09 GB/s per link. Utilizing direct FPGA-to-FPGA transfers reduces the required memory bandwidth by up to 75%.