Depending on the amount of data that would be needed on the plan it may be cheaper to get two single line plans, as some carriers will do group save discounts.
It really depends on which carrier plans you are looking at, whether you already have a phone and how much data, minutes and texts you are looking for.
As a general rule though - the more lines you need, the cheaper it will be to get a multi line or share plan, versus buying separate lines, particularly if you need a lot of data.
Some carriers such as Metro PCS offer a discount for buying more than one line.
So really in the end it can be cheaper or more expensive for a share plan, all depending on your needs.
The best way to start comparing is to use our wireless plan search and play around a little bit between single line searches and multi line searches.