When you switch to the tethering plan, you will no longer have the unlimited plan. You will only be able to use the 2GB plan and anything over 2GB will be charged extra.
Most likely since the unlimited plan is no longer offered, you won't be able to switch back to the unlimited plan once you no longer want the 2GB tethering plan.
You can't keep both plans. MyWi still uses data and there is no way to hide that fact.