There are too many ways that my answer could be misinterpreted.
1) You say "Stress Range versus Cycles". The (ASME) curves we ship with CAESAR II are Stress Amplitude versus Cycles. You can do either because we also employ a multiplier in that file. That multiplier converts your local unit dimension to CAESAR II native dimension (e.g. MPa to psi). That maultiplier can also be used to move from amplitude to range (or range to amplitude). That multiplier may also include a factor of safety, or, in your case a stress "adjustment".
2) You say in a) "a stress multiplier of 2". Is this to address your condition (I don't know how to better put this) that the "real SIF" for a girth butt weld is 2, not 1? If so, then, yes, that doubling effect can be applied with this multiplier. BUT, I cannot say the number you type in is "2". Note that for US units, the FAT file contains the value "0.5" is used so in that case, a value of "2" would quadruple.
3)In b), I guess you could double each and every SIF but your would have to define all your welds too. In concept, I think it would be OK but in application it's sloppy and error prone.

I have to again emphasize - give it a try. You will see how your entered srtess data (range or amplitude) is applied in the static (or harmonic) analysis. Yes, it's confusing but you can quickly confirm how your entered data s used by looking at the output. If the output is off, change that multiplier in the FAT file.

For those of you whoa re still reading this, I show two FAT files below. Note how the "Stress Multiplier" is used.

Here's out FAT file for low strength steel:

* ASME SECTION VIII DIVISION 2 FATIGUE CURVE
* FIGURE 5-110.1
* DESIGN FATIGUE CURVES FOR CARBON, LOW ALLOY, SERIES 4XX,
* HIGH ALLOY AND HIGH TENSILE STEELS FOR TEMPERATURES NOT
* EXCEEDING 700 F
* FOR UTS <= 80 KSI
*
0.5000000 - STRESS MULTIPLIER (PSI); ALSO CONVERTS AMPLITUDE TO FULL RANGE
*
10 580000.0
100 205000.0
1000 83000.0
10000 38000.0
100000 20000.0
500000 13500.0
1000000 12500.0
0 0.0

And here's one I built to mimic Markl:

* using Sc=Sh=20ksi with f in Eqn 1a
* will give the following fatigue curve
* use this with multiple sources of strain ranges
*
1.0000000 - STRESS MULTIPLIER (PSI); these are ranges
*
10 36000.0
3000 36000.0
7000 30637.0
10000 28528.0
15000 26306.0
20000 24835.0
50000 20677.0
100000 18000.0
0 0.0
*