How to set multiple values inside an if else statement?

If you have more than one statement in a if condition, you must use the BEGIN ... END block to encapsulate them.

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
BEGIN
 SET @test1 = 'test1'
 SET @test2 = 'test2' 
END
ELSE
BEGIN
 SET @test1 = 'testelse'
 SET @test2 = 'testelse'
END

Use BEGIN and END to mark a multi-statement block of code, much like using { and } in other languages, in which you can place your multiple SET statements...

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
BEGIN
    SET @test1 = 'test1'
    SET @test2 = 'test2'
END
ELSE
BEGIN
    SET @test1 = 'testelse'
    SET @test2 = 'testelse'
END

Or, use SELECT to assign values to your variables, allowing both to be assigned in a single statement and so avoid requiring the use of BEGIN and END.

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
    SELECT
        @test1 = 'test1',
        @test2 = 'test2' 
ELSE
    SELECT
        @test1 = 'testelse',
        @test2 = 'testelse'

The behavior makes sense since your first trial will be split like this:

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
SET @test1 = 'test1'

SET @test2 = 'test2' 

ELSE
   SET @test1 = 'testelse'

and ELSE will fail since it does not belong to any IF statement.

Consider doing like this:

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
BEGIN
   SET @test1 = 'test1'
   SET @test2 = 'test2' 
END
ELSE
BEGIN 
   SET @test1 = 'testelse'
   SET @test2 = 'testelse'
END

If you have multiple statements after the IF you have to use begin and end (similar to accolades in c#, for example).

IF ((SELECT COUNT(*) FROM table WHERE table.Date > '2016-03-20') > 10)
BEGIN
   SET @test1 = 'test1'
   SET @test2 = 'test2' 
END
ELSE
BEGIN
   SET @test1 = 'testelse'
   SET @test2 = 'testelse'
END