I host a very similar Burnham with my pellet boiler.
I also chose to plumb in series just as you are thinking so when (not if) the pellet boiler fails the NG picks up the load. It was also easier - I use Teckmar ODR single boiler controls on each and am sure to set the NG control on a looser ODR curve than the pellet boiler for a lead follow team.
I chose to heat the NG boiler with the pellet boiler - just a way to look at it -and placed this contraption bypass for protection and balance - note the supply from the corn boiler feeds into the return of the existing NG boiler and the return of the NG boiler routes to the return or the corn boiler with a balance/tempering valve.
There is a control on the corn boiler that will halt primary loop pumping when supply drops below 145 - that allows the pellet boiler to idle in a non condensing mode. But when the house load calls, a small portion of energy is drawn away from the pellet boiler to feed the load - modulated by that balance valve. When the pellet boiler is heat soaked and on hi fire the primary loop pump defeats this open crossover and full temp water is sent to the load. If the pellet boiler cannot keep up to the load - and I usually have the burner off on the NG appliance unless its cold or I plan on being away for more than 8 hours - the Teckmar launches the gas boiler smoke damper that allows the gas train to fire and pick up the primary loop energy level. With out that Efical damper, there would be significant energy lost up the NG boiler stack.
Hope that helps - should work with coal as well and corn and pellets!
ps -- please forgive the mix of galv and black fittings and scrap pipe - I did this on the cheep mostly with things I had laying around.