Can we use two thunderbolt 1 ports as two inputs into a 4k monitor
This is only possible on monitors that require two cables to run at full resolution. As far as I know this is only the Dell 5K monitor (http://www.dell.com/en-us/work/shop/cty/dell-27-ultrasharp-ultra-hd-5k-monitor-up2715k/spd/dell-up2715k-monitor).

Even if you were to connect a 4K monitor that supports multiple inputs to your MacBook, it would connect as two separate monitors with two separate spaces, so you'd see the menubar twice.