Here's another idea (somewhat unconventional). You could create a user defined function to implement the second range filter (in Cassandra 2.2 and newer).
Suppose you define your table like this (shown with ints instead of timestamps to keep the example simple):
CREATE TABLE tasks (
p int,
task_id timeuuid,
start int,
end int,
end_range int static,
PRIMARY KEY(p, start));
Now we create a user defined function to check returned rows based on the end time, and return the task_id of matching rows, like this:
CREATE FUNCTION my_end_range(task_id timeuuid, end int, end_range int)
CALLED ON NULL INPUT RETURNS timeuuid LANGUAGE java AS
'if (end <= end_range) return task_id; else return null;';
Now I'm using a trick there with the third parameter. In an apparent (major?) oversight, it appears you can't pass a constant to a user defined function. So to work around that, we pass a static column (end_range) as our constant.
So first we have to set the end_range we want:
UPDATE tasks SET end_range=15 where p=1;
And let's say we have this data:
SELECT * FROM tasks;
p | start | end_range | end | task_id
---+-------+-----------+-----+--------------------------------------
1 | 1 | 15 | 5 | 2c6e9340-4a88-11e5-a180-433e07a8bafb
1 | 2 | 15 | 7 | 3233a040-4a88-11e5-a180-433e07a8bafb
1 | 4 | 15 | 22 | f98fd9b0-4a88-11e5-a180-433e07a8bafb
1 | 8 | 15 | 15 | 37ec7840-4a88-11e5-a180-433e07a8bafb
Now let's get the task_id's that have start >= 2 and end <= 15:
SELECT start, end, my_end_range(task_id, end, end_range) FROM tasks
WHERE p=1 AND start >= 2;
start | end | test.my_end_range(task_id, end, end_range)
-------+-----+--------------------------------------------
2 | 7 | 3233a040-4a88-11e5-a180-433e07a8bafb
4 | 22 | null
8 | 15 | 37ec7840-4a88-11e5-a180-433e07a8bafb
So that gives you the matching task_id's and you have to ignore the null rows (I haven't figured out a way to drop rows using UDF's). You'll note that the filter of start >= 2 dropped one row before passing it to the UDF.
Anyway not a perfect method obviously, but it might be something you can work with. :)