The height achieved with 2 x 1600g balloons lifting 2Kg will be the same as 1 x 1600g balloon lifting 1Kg with half the helium. The height you can reach is dependent on the weight lifted - in general the heavier your payload the lower your balloon will burst. The highest amateur HAB altitude records were all achieved with very light payloads (sub 100g) - see the records section here:
You can use this burst calculator to estimate the effect of changing various parameters:
For example - the difference between the bursting altitudes for a HY-1600 balloon for 1Kg and 100g is very roughly about 10,000ft (depending on a number of factors).
In you 2nd topic - if you constrain the balloon with an external material the balloon will not burst but stop ascending at the point it becomes neutrally buoyant with the external atmosphere. This will occur lower than the burst altitude. Of course the additional weight of the material will reduce the altitude too.