Distribute two SELECTs with a given ratio

I have two tables with the following data

table "group1":

id  | sequenceNo
----+-----------
101 | 1
102 | 2
103 | 3
104 | 4
105 | 5

table "group2":

id  | sequenceNo
----+-----------
201 | 1
202 | 2
203 | 3
204 | 4
205 | 5

I have a given ration of 3:1 which should build a mix of the groups.

The result would be:

id
--
101
102
103
201
104
105

Ideally the mixing stops when one of the groups is empty.

I've implemented a solution for the problem as an OO-program. However, I am curious if there is also a simple SQL-only solution.

Many thanks,

Maik

via Chebli Mohamed