1.No.In a table only one primary key.we have to use the
behavior of primary key i.e in a colon to set unique key and
not null.
2.Cluster index is the leaf node directly consist a values.
and non cluster index is the leaf node consists address of
value at same task or heap. 
3.No

1.no we can't set more than one primary key for a table
3. yes we can write a query like select 8 from table1,table2
this is generally used in case of cartesian product

1. A table can only have one primary key.
2. Answer #1 is right. To add it, primary key is clustered
index and when we index a column (non-primary key) it is
non-clustered index.
3. Yes this will act almost as OUTER join

1.We cant see more than one primary key for the table,
however we could create composite index using clustered
index in a table.
2.Clustered index - Order the data in the physical order.
Non clustered index does not order the data
3.We can select both the table using that script which u
mentioned.

1)No We can't have more than 1 primary key. Even if you try
to save it will pop an error message and nothing will be stored.
2)Clustered Index is attached with Unique. A table can have
1 and only 1 Clustered Index and it will be mostly created
on the Primary Key of the Table. 
Non Clustered can be up to 254(not sure about the
number)created for a single table.
3)You can definitely select from 2 tables but all the
columns will be merged. Please check how you want the data
from those 2 tables

1. No.

2.when create primary key on a table clustered index 
automatically created.1 clustered index is possible for a 
table.
when we create a unique key on table Non clustered index 
automatically created. 253 non clustered indexes are 
possible for a table.

3. yes (its not recommended)