| 1. Impedance (measured in Ohms) -- for a number of cabinets wired in parallel (the usual situation), the impedance decreases as the number of cabinet increases. Total impedance = 1/[(1/R1) + (1/R2) + ...
This is easy when the impedances of the cabinets are the same, e.g. 2 x 8 Ohm cabinets = 1/[(1/8) + (1/8)] = 4 Ohms.
2. Power handling -- the "wattage" figure quoted on the cabinet's spec sheet and/or back panel is the maximum power it can handle before the voice coil suffers heat damage (i.e. "thermal rating") and bears little real relationship to the power the speaker will handle before it distorts/farts, etc, although this is typically about half what is cited. The power output from the amp will be divided between the cabinets according to the impedance they present to the amp's output; i.e. if both cabs have the same impedance (e.g. 8 Ohms), then each will receive half the amp's power output (i.e. in your case ~225 Watts to each cabinet). In practice, you can pretty much ignore the power rating stamped on the cabinet, and listen for the nasty sounds a speaker makes before it craps out, then back off the volume, and/or the bass end of your EQ. Bass frequencies require much more of a speaker in terms of coil/cone motion and therefore heat generation. Much of your "tone" comes from the middle of the frequency spectrum anyway, so not too much of a big deal to back off the low end a bit, and it will allow you to get much more volume from your speakers.
More information than you asked for, perhaps, but I hope helpful nonetheless.
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Christopher 401T / Gage Realist Soundclip / Fishman Pro-EQ Platinum Bass / fdeck HPF-Pre Series 2
NS Design CR4M EUB / TC Electronic RH450 & Markbass F1 / BFM Jack 112 | 
