Tesla products do not support SLI. However, multiple Tesla products will allow for greater overall performance provided the applicattion is multi-GPU aware.
If you have multiple PCI-Express x16 slots, you can put a Tesla C870 in each slot. You have to write your application code so that it is multi-GPU. There is an example in the CUDA SDK for multi-GPU Monte Carlo code that demonstrates how to write multi-GPU code.
Similarly, if you have multiple PCI-Express x8 or x16 slots, you can put in host adaptors in each slot and connect multiple Tesla D870 desksides.