Dual (elec and wood) furnaces. Route air ducting in series or parallel? Any preferences?

My contuinuing saga on the wood furnace is now this: my hvac buddy says I should install in series, having the hot air from the wood furnace (which has a blower) feed into the cold air return of the existing electric furnace. Also to re-route the cold air return from the electric into the wood furnace.
Does anyone have any opinions on this compared to doing it in parallel?
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