Bad news - there appears to be no stacking.
Made a 6 wizard/1 cleric character.
Two testing modalities -
1) Use lesser death aura on a summoned skelly
2) Use necrotic touch on a summoned skelly
Lesser death aura produces a d4 of negative damage. With 25% boost, one would expect a spread of 1,2,3,5 since the software tends to round down. So the only value that goes up is the 4. If the cleric stacks, 35% would let 3 go up to a 4, producing 1,2,4,5. Both with and without the cleric life magic I, the spread on the aura was 1,2,3,5.
Necrotic touch was averaged over 100 data points, with crits excluded. The average with just PM I was 29.47, PM I and Life Magic I was 30.21. That's about 2.5% difference, and probably within expected variations for a sample space this small (I saw 200 out of 6^7* or 279936 possible rolls). Using an average of all points, divided by 1.25 to remove the boost, then 7 to get the average per die, we get 3.41 per die, or slightly lower than the expected 3.5, but still close enough I'm not too worried.
*Side note - necrotics go up with character level, not wizard level, much like the monk healing finisher. So even splash wizards can have full strength necrotic effects.
Data points
Code:
PM PM+Cler I
28 22
28 41
31 36
26 22
31 35
33 35
33 22
26 28
27 37
23 21
26 25
30 28
31 37
41 28
25 26
18 28
37 20
32 26
27 38
21 23
22 30
32 35
41 36
30 26
27 27
32 37
26 26
27 32
26 30
33 32
33 30
27 16
26 20
42 40
32 26
40 21
33 23
31 36
32 31
32 28
23 16
33 36
25 32
26 35
25 32
31 31
27 42
18 40
28 35
28 25
28 41
28 32
22 25
21 33
27 31
31 36
23 30
33 33
28 26
31 23
27 26
36 23
26 23
46 35
36 25
26 33
31 27
35 32
26 31
31 25
33 27
31 28
25 32
23 37
26 41
42 27
40 41
35 30
31 32
28 26
27 31
35 25
21 40
32 36
21 28
18 26
31 41
37 22
35 33
25 30
32 21
27 32
28 23
25 27
31 33
26 35
36 35
30 36
32 41
30 31
29.47 30.21