vgl. ein beschränkt Handlungsbevollmächtigter. 'Die (jeweilige) Handlungsvollmacht kann auch mehreren gemeinsam erteilt werden...'

'It`s the combination of the two terms that confuses me, as "mancomunado" and "solidario" are the exact opposites.' > not exactly: mancomunado means joint only and solidario means joint *and* several.

See the second weblink:

'[either...] joint or joint and several
Explication :
They are similar but not the same

Rebecca distinguishes those words in her lexicon. See example below:

Administración por un consejo rector o un administrador único, dos administradores mancomunados o dos administradores solidarios Management by a management board or a sole director, two joint directors or two joint and several directors

Administrador único, administradores mancomunados, administradores solidarios o consejo de administración > Sole director, joint directors, joint and several directors or board of directors

Extinción de la obligación por el pago hecho por uno de los deudores solidarios > Discharge of the obligation through payment made by one of the joint and several debtors.'

Note that, in English Common law, trustees like personal representatives testamentary executives of a deceased can give a receipt and sign individually for a sale of chattels but two or three must sign jointly for land

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Note added at 3 heures (2020-09-25 11:24:03 GMT)
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personal representativese.g. testamentary executors of a BrE deceased person AmE decedent..