What you want does not exist, nor will ever in current mainstream platforms. There are implementations that advertise themselves as 16x/16x, but they use a special switch. You may want to read
this (And an old Thread
here).
Haswell supports up to three PCIe Slots coming from the 16 PCIe lanes of the Processor itself, and can run in 16x, 8x/8x or 8x/4x/4x modes. Most Motherboards employs bridges that can simply route the lanes from one slot to another depending on if there is a card on it or not, I don't recall seeing any that instead of using bridges has them fixed wired. Fixed 8x/8x should be good and viable because is good enough for vast majority of users (I think still no Video Card saturates PCIe 3.0 8x Bandwidth, at least on previous generation they still weren't there) and removes some added complexity and cost due to the bridge chips.
Motherboards that advertise to have 16x/16x, uses a special switch between Slot and Processor that provides 16 lanes for each. HOWEVER, this switch is limited by the fact than Haswell still has only 16 lanes. The only thing than the switch does, is reroute them on-the-fly based on GPU needs. This means that if you needed at some moment 12 lanes worth of bandwidth at one slot and the other card is Idle, with this switch you would not have a bottleneck that you would on 8x/8x because it rearranges that on-the-fly. However, such scenarios are unlikely to happen, and you still have the 16 lanes limit from Haswell, so if you needed more, you would still have a bottleneck and the switch will be good for nothing. Actually, it is already good for nothing because it adds latency and overhead and no performance benefit, because as most Video Cards can't saturate PCIe 3.0 8x Bandwidth, the scenario where it could be useful shouldn't be possible on the first place.