Grafana - combine multiple SELECT from different measurments

Question

I'm totally new here, I tried to find solution and I found this:
Influxdb and Grafana combine multiple SELECT

but this works fine if we have different fields from the same measurement, but this does not work (for me) when adding values from different measurements. What am I doing wrong here? Could you please take a look into this? Updated query - with proper interval - still does not work.

SELECT ( sum("valFromA") + sum("valFromB") )  
AS "addedVal"   
FROM (  
    SELECT mean("valueA") AS "valFromA"  
    FROM "DSA"   
    WHERE "hostname" = 'host' AND $timeFilter GROUP BY time(15m) fill(null)  
),  
(      
    SELECT mean("valueB") AS "valFromB"  
    FROM "DSB"   
    WHERE "hostname" = 'host' AND $timeFilter GROUP BY time(15m) fill(null)  
)  
GROUP BY time(15m) fill(null)  

Nothing is drawn in this case. This also show 2 counters under graph DSA.addedval DSB.addedval

BTW - when i split this queries to separate ones - it returns data. Combined as above shows empty graph. e.g.

Single select is ok:

SELECT ( sum("valFromA") )  
AS "addedVal"   
FROM (  
     SELECT mean("valueA") AS "valFromA"  
     FROM "DSA"   
     WHERE "hostname" = 'host' AND $timeFilter GROUP BY time(15m) fill(null)  
)
GROUP BY time(15m) fill(null)  

Grafana 6.4.4 + Influx 5.1.0 Do I need to do something in Influx for this? I'm asking because I've access to grafana only, but I may ask our administrators.

UPDATE: I really tried to put this into proposed "select" by Akina, but I didn't succeed. I don't want to paste here my trials, because grafana does not understand clause UNION ALL. So maybe it is possible but in influx not directly from visualization (graph) query definition.

This is what I'm trying to do:

SELECT ( (sum("opt12") / sum("dst45")) * 100 )
AS "Ratio" 
FROM (
    SELECT mean("ratioopt12") AS "opt12"
    FROM "ottb.gen.11" 
    WHERE "hostname" = "serv1"  AND $timeFilter GROUP BY time(15m) fill(null)
),
( 
    SELECT mean("ratiodst45") AS "dst45"
    FROM "ottb.gen.14" 
    WHERE "hostname" = "serv1" AND $timeFilter GROUP BY time(15m) fill(null)
)
GROUP BY time(15m) fill(null)

Really have no idea what should I replace "field" with in this case, and what about group by time(15m) here?

SELECT sum(summ), field FROM (
   SELECT sum(val_1), field AS summ FROM tableA GROUP BY field 
   UNION ALL 
   SELECT sum(val_2), field FROM tableB GROUP BY field
) GROUP BY field    

Any help would be appreciated.

Regards,
Mike