Assume you have a u=3BB/hr (mean) and a var=225BB/hr per table, and consequently a StdDev=15BB/hr.
If you are two-tabling, You would have a u=6BB/hr, var=450BB/hr, andStdDev=21.21BB/hr

Note that by playing 2 tables at half the limit, you can reduce your standard deviation per hour without reducing your win rate, and hence greatly reduce your risk of ruin for a given bankroll, or cut your bankroll requirement in half for a given risk of ruin. This assumes that you can make the same number of BB/hr at each table at the lower limit as you did at the higher limit.