small prob in join?

My answer is very similar to @Kev Riley's (so much so that I didn't post it once I saw his...), but with your latest requirement, here goes:

declare @t1 table
(
    resid char(1),
    mid tinyint,
    s int
)

declare @t2 table
(
    mid tinyint,
    val int
)

insert into @t1(resid, mid, s) values
    ('a', 2, 10),
    ('b', 2, 10),
    ('a', 2, 10),
    ('b', 3, 20),
    ('c', 3, 20);

insert into @t2(mid, val) values
    (2, 10),
    (3, 20);

select
    sum(r.resids) as numResids,
    SUM(t2.val) as summation
from
    @t2 t2 
    inner join (select mid, COUNT(resid) as resids from @t1 group by mid) r on r.mid = t2.mid
where
    t2.mid in (2, 3);

The load statements are specific to SQL 2008 and later, but the select statement (which is the important part here) will work in previous editions.

If the second table already has the sum(val) aggregated by mId, then you need to join differently

declare @MyFirstTable table (resid char(1), mId int, [sum] int)
insert into @MyFirstTable select 'a',2,10
insert into @MyFirstTable select 'b',2,10
insert into @MyFirstTable select 'a',2,10
insert into @MyFirstTable select 'b',3,20
insert into @MyFirstTable select 'c',3,20

declare @MySecondTable table (mId int, val int)
insert into @MySecondTable select 2,10
insert into @MySecondTable select 3,20

select
    firsttable.countval,
    secondtable.val
from
    (
    select
       mId, COUNT(resid) as countval
    from
       @MyFirstTable 
    where mId in (2,3)
    group by mId
    ) firsttable
join @MySecondTable secondtable on firsttable.mId = secondtable.mId

gives

countval    val
----------- -----------
3           10
2           20

(2 row(s) affected)

Edit : OP has clarified the output requirements

Try this

select
(
select
    COUNT(resid) as countval
from
    @MyFirstTable 
where mId in (2,3)
),
(
select SUM(val) 
from @MySecondTable
where mId in (2,3)
)