Hi,

1. Each delegation is separate and will expire 90 days after their individual creation date.
2. It's generally better to login before purchasing, because then we can maintain records of the purchase that you can review later. It's particularly useful for delegations, because we can offer an interface for extending delegations you purchased while signed in.
3. Each delegation purchased is separate, they are not connected to previous delegation purchases.
4. Since each delegation is separate, you could login and purchase a new delegation now. In this case, you will have the sum of the Steem Power from both delegations until the first one expires. Or you can wait till the first one expires before purchasing the second one if you don't want the temporary Steem Power increase.