没理由不同的GPU不可以一起工作，最多是发挥不了最大性能而已。

SLI allows two, three, or four graphics processing units (GPUs) to share the workload when rendering real-time 3D computer graphics. Ideally, identical GPUs are installed on the motherboard that contains enough PCI-Expressslots, set up in a master-slave configuration. All graphics cards are given an equal workload to render, but the final output of each card is sent to the master card via a connector called the SLI Bridge. An example, in a two graphics card setup, the master works on the top half of the scene, the slave the bottom half. Once the slave is done, it sends its render to the master to combine into one image before sending it to the monitor.
The SLI bridge is used to reduce bandwidth constraints and send data between both graphics cards directly. It is possible to run SLI without using the bridge connector on a pair of low-end to mid-range graphics cards (e.g. 7100GS or 6600GT) with Nvidia's Forceware drivers 80.XX or later. Since these graphics cards do not use as much bandwidth, data can be relayed through just the chipsets on the motherboard. However, if there are two high-end graphics cards installed and the SLI bridge is omitted, the performance will suffer severely as the chipset does not have enough bandwidth.
Configurations currently include:
2-Way, 3-Way, and 4-Way SLI. Uses two, three, or four individual graphics cards respectively.Two GPUs on one graphics card. Examples include the GeForce GTX 590, GeForce GTX 690 and the GeForce GTX Titan Z. This configuration has the advantage of implementing Two-Way SLI, while only occupying one PCI-Express slot and (usually) two expansion I/O slots. This also allows for Four-Way SLI using only two cards (which is referred to as Quad SLI).