Find Max Serial Number From 2 Tables

I need a query to find max serial number by comparing two different tables.

Here is my requirement

I am having two tables named Table1 and Table2. Each tables having more than 30,000,000 records.

I want a simple query to find Max srno from two tables.
For example
If Table1 max is 245 where partno=2 and ano=2
and Table2 max is 343 where partno=2 and ano=2
Then 343 is max serial no

If Table1 max is 435 where partno=2 and ano=2
and Table2 max is 34 where partno=2 and ano=2
Then 435 is max serial no


I used this query but its taking more time
 select max(v.MaxSrNo) from
((select max(MaxSrNo) as MaxSrNo from Table1 where partno=@partno and ano=@ano)
union all (select max(MaxSrNo) from Table2 where partno=@partno and ano=@ano)) as v 

Pls give me a simple query to find max srno.