Usually you want to go with the best SINGLE card you can. SLI or Crossfire doesn't always scale 100% so adding a second GPU doesn't always automatically double performance. Plus, the load (electrical and thermal) on a system increases quite a bit more as you're doubling power usage without getting that 100% performance increase.
Besides, we know almost nothing about Pascal. Allegedly with will feature HBM, and if it's as good as their highly efficient Maxwell architecture it will be quite a powerhouse. As long as it's not twice the price of a pair of GTX980s, while still increasing performance, then you're better off waiting.
Heck, you're better off waiting anyhow. Look at the situation with the Radeon 3xx series vs 2xx series. Not a big jump in capabilities while the 2xx series plummeted in price. That made the 2xx series a better buy. That could also happen here. Think about it, if the new Pascal series is good, then you want to go Pascal. If it's marginal, you can pick up the 'last gen' hardware at a discount as some people will still upgrade and sell their used hardware. Either way, you lose nothing by waiting.
tldr;
If you wait:
if Pascal underwhelms, you can SLI your 980, possibly at a discount since 980s are last gen.
if Pascal is great, you can upgrade to a single gpu.
If you buy now:
You'll pay full price for almost last gen hardware.
You might have to pay again if Pascal is as good as it might be.
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